1. Field of the Invention
The present invention relates to a zoom lens system, an imaging apparatus, and a method for varying a focal length of the zoom lens system.
2. Related Background Art
A negative-leading type zoom lens system in which a negative lens group is disposed to the most object side of the zoom lens system has been widely used as a wide-angle zoom lens system because of advantages such as a short minimum shooting distance and ease to make the focal length short in the wide-angle end state. Since power distribution of the negative-leading type zoom lens system becomes a retrofocus type having, in order from an object, negative-positive in a wide-angle end state, and a telephoto type having positive-negative in a telephoto end state, it becomes possible to make the focal length longer in the telephoto end state.
There has been proposed a negative-leading type zoom lens system that includes, in order from the object, a first lens group having negative refractive power, a second lens group having positive refractive power, a third lens group having negative refractive power, and a fourth lens group having positive refractive power, and carries out zooming by varying distances between adjacent lens groups (for example, Japanese Patent Application Laid-Open No. 5-019170).
Although the zoom lens system disclosed in Japanese Patent Application Laid-Open No. 5-019170 has a wide angle of view and a given zoom ratio and is small and compact with a low manufacturing cost, it is difficult to obtain good optical performance over entire focal length range, so that higher optical performance has been expected.